Hi Marley,
If you are talking about the expensive in-ear thermometers - don't bother. I got one as a gift and it was a total waste. The ear piece is not small enough for a baby and can only be used on bigger children/adults.
My child has had numerous fevers (first one at 6 weeks) and the most accurate method is actually to take a rectal temperature. I never did this, but used a cheap "vicks brand" under arm thermometer" (R60) through all our fevers. Yes, the temperature will vary with a few point of degrees, but not one entire degree if taken 3 times in a row - but the bottom line is you will have a general idea if a fever is present or not (most likely you would know in any case since fussiness is usually present or other symptoms). With every fever I have been able to feel it on my child's body too since she would be extremely hot. (This is not an accurate method, though practise has made me know the difference between a child who is hot due to a hot summers day or from a fever).
While my child was in hospital they mainly used and underarm digital thermometer as well.
Remember about educating yourself about how to take the reading since depending on where you take the reading, oral, rectal, under arm will give different results. For example, an under arm thermometer will give a lower reading where you have to add around 0.5 - 1.00 degree to your reading to know if it is a fever or not. My child for instance has a normal temperature of 35.5 on our thermometer, which is quite low. Whenever she is higher than 36 I know that is the start of a fever, although in medical terms that still falls into a normal temperature, my child's body temperature seems to be lower or my thermometer is taking a consistent lower temperature.
Hope this helps!